The invention relates to respiratory-analysis mattresses and systems, and to methods of use thereof.
The invention further relates to the measurement of respiratory, cardiac and other movement related functions in patients suffering from a range of respiratory syndromes, including the disordered breathing associated with Cheyne Stokes syndrome, anaesthetic induced partial respiratory obstruction and sleep apnea.
Sleep apnea is a respiratory syndrome known to be present in about 8% of the adult male human population and 4% of the adult female human population.
The syndrome manifests itself as the repetitive cessation of, or large reduction in, breathing while the patient is asleepxe2x80x94respectively termed apneas and hypopneas. Apneas may be divided further into central apneas, where the cause of the apnea is the failure of the nervous system to activate the muscles responsible for respiration, and obstructive apneas, where the patient tries to breath but is prevented from doing so by the temporary collapse on inspiration of his or her upper airway. The reasons for such collapses are not completely understood but may include a loss of tone in those muscles which hold the airway open plus an anatomical disposition towards a narrow upper airway.
Prior to treatment the syndrome must be diagnosed. Conventionally, this is performed by an overnight study in a specialised sleep clinic, connecting the patient to electrophysical and respiratory measurement equipment to monitor physiological variables such as the electroencephalogram, blood oxygen saturation, heartrate, chest wall movement, and respiratory air flow during the various stages of sleep.
The attachment of the such monitoring equipment requires skilled staff and is often disruptive to the patient""s sleep. Furthermore, the recording of all the physiological variables requires considerable computing power and the subsequent analysis, although assisted by computer, still requires considerable attention by the staff.
Monitoring of the patient""s sleep in the patient""s home traditionally uses a simplified form of the above-mentioned equipment which still may be complex and disruptive to the patient""s sleep.
The measurement of less disruptive variables which correlate well with the traditional ones has been pursued as a way of making such sleep studies simpler to perform and less disruptive to the patient.
A device used in this area is the Static Charge Sensitive Bed (SCSB) described in U.S. Pat. No. 4,320,766 (Allihanka et al). U.S. Pat. No. 4,320,766 describes a mattress which outputs a single electrical signal that varies with the patient""s movement. By suitable electrical filtering of the movement signal indications of body movement, respiration, snore and heartbeat are produced for subsequent display.
The SCSB principle was extended by Crawford and Kennard in their published UK patent application, GB 2 166 871 A (1984), for a Respiration Monitor. Here, strips of polyvinylidene fluoride (PVDF) were assembled in a common, parallel connection in order to give area coverage of a patient""s respiratory movement. PVDF is a piezo-electric plastics material readily available in strips and sheets of minimal thickness.
A PVDF sensor has also been used in a device described by Siivola [Siivola J., (1989) New noninvasive piezoelectric transducer for recording of respiration, heart rate and body movements. Med. and Biol Eng. and Comput. 27, 423-424].
The clinical use of the SCSB is extensively described in the PhD thesis of Dr O. Polo (Dept of Physiology, University of Turku, Finland) republished as a supplement in Acta Physiologica Scandinavica Vol 145, Supplementum 606, 1992.
A PVDF film based device for detecting and recording snoring is also described in International Publication No. WO 96/36279 (Sullivan).
A limitation of the SCSB is that because of its inherent planar construction it cannot be used to localise the source of the movement it detects. Likewise, the above-mentioned devices also generate minimal spatial information. A major consequence of this is that the outputs of the said devices vary considerably with patient orientation. This limits the accuracy of information that can be derived from them.
The invention seeks to provide a respiratory-analysis mattress and system and associated method which overcome or at least ameliorates some of the deficiencies of the prior art.
According to the invention there is provided a mattress for monitoring patient movement for a patient lying on the mattress, the mattress including at least a plurality of independent like movement sensors for measuring movement at different locations on the mattress to generate a plurality of independent movement signals.
Preferably, the respiratory-analysis mattress includes the range three to ten such movement sensors.
Preferably, the movement sensors are formed by piezoelectric elements, for example polyvinylidene fluoride (PVDF) sensor strips.
Conveniently, at least some of said piezoelectric elements are arranged to measure lateral strain across the mattress, and preferably substantially parallel to each other.
In one embodiment of the invention, the mattress comprises a compressible filling surrounded by an outer (not necessarily outermost) layer, and the piezoelectric elements are attached, preferably by adhesive, to an inside surface of the outer layer.
Alternatively, the elements are laminated between two thin conformable elastic sheets.
In another embodiment of the invention the mattress takes the form of a thin movement-sensitive sheet, in which each piezoelectric element is enclosed within a waterproof envelope of material.
Such a movement-sensitive sheet can be mounted on a carrier sheet adapted to be placed over a conventional mattress.
Conveniently, each piezoelectric element is connected to a separate transition connector, which may for example be a printed circuit board (PCB), in such a manner as to avoid strain between the element and the transition connector during use of the mattress.
Alternatively, the piezoelectric elements can be connected to a single bus transition connector, which may for example be a single bus board.
In a further embodiment of the invention, said piezoelectric elements are integrally formed from a single composite sheet of piezoelectric material.
The piezoelectric material may be PVDF.
In this case, the piezoelectric elements can be formed by forming a series of staggered parallel cuts in a sheet of piezoelectric material, in such a way that each piezoelectric elements is formed between two adjacent such cuts, and folding each piezoelectric element through an angle so that the piezoelectric elements become spaced from each other, while remaining integrally connected together by a connecting portion of said sheet of piezoelectric material.
Conveniently, said angle is substantially 90xc2x0.
In one embodiment, a strengthening portion of said sheet of piezoelectric material is folded, and attached to, said piezoelectric elements in order to hold the strips in position.
Preferably, each piezoelectric element is provided with upper and lower metallised surface layers which are arranged not to overlap on the folded portion of the element, thus ensuring that any strain on the folded portion of the element does not contribute to the electrical signal produced by the element.
The invention also provides a respiratory-analysis system comprising a mattress as described above connected to processing means for receiving and processing said movement signals to derive said respiratory variable(s). The diagnostic variables can include respiratory rate, respiratory phase, respiratory effort and maximum respiratory rate.
Preferably, the output signals from each movement sensor are separately and independently processed. The resulting processed information may subsequently be combined by the computing means during further processing.
Preferably, each piezoelectric strip is connected to the computing means via a sensor buffer comprising an operational amplifier operating as a charge amplifier, wherein the input of the operational amplifier is protected against high voltage transients, which may be produced by the piezoelectric strip, by means of a resistor connected in parallel across the two inputs of the operational amplifier, said resistor having an impedance which is large compared with the input impedance of the virtual earth of the charge amplifier.
In addition a further resistor, having an impedance which is small compared to the output impedance of the piezoelectric strip, can be placed in series with the input of the operational amplifier.
Alternatively, two diodes can be connected in parallel, and in opposite directions, across the two inputs of the operational amplifier in order to limit the maximum voltage applied to the inputs of the operational amplifier.
The invention further provides that the processing means combined one or more of said respiratory variables to give one or more derived diagnostic variables.
The derived variables can include apnea classification, snore or obstructed breathing.
The invention further provides a method for monitoring patient movement for a patient lying on a mattress, the method comprising the steps of: measuring displacement due to body movement at a plurality of independent positions length-wise of a portion of the body to derive independent signals representative of respective individual body displacement at said positions.
The invention yet further provides a method for monitoring at least one respiratory variable for a patient lying on a mattress, the method comprising the steps of: measuring displacement due to body movement at a plurality of independent positions length-wise of a portion of the body to derive independent signals representative of respective individual body displacement at said positions; and processing said movement signals to derive said respiratory variable(s).
The invention yet further provides a respiratory analysis system for monitoring at least one respiratory variable for a patient, comprising: input means to receive a plurality of signals indicative of patient movement; and processor means to process said receive signals to derive said respiratory variable(s).
The invention further provides the method may further comprise detecting the occurrence of non-respiratory sudden body movements by at least detecting high frequency components of the movement signals in the absence of diagnostic signals indicating obstructive or central apnea occurrence.
The method may further comprise determining the degree of obstructed breathing present by at least detecting the ratio of respiratory effort signal to respiratory displacement signal plus optionally the amplitude of the snore signal.
The method may further comprise controlling the output treatment pressure of a Continuous Positive Accuracy Pressure (CPAP) treatment machine on the basis of information obtained in preceding steps of the method.
Such control either occurs in real-time, that is as the respiration is being monitored, or retrospectively, where accumulated respiratory data is processed to determine the treatment pressure subsequently to be programmed into the CPAP machine.
The method may further comprise storing video information relating to the patient only for those periods during which an apnea has been detected as explained above.
The method may further comprise outputting any of the information obtained in preceding steps of the method to a display or printer, or to an analogue physiological input channel of a polygraph in such a way as to display information in alphanumeric form on the display of the polygraph.